Improve revision slider functionality

Follow-up from https://github.com/Automattic/simplenote-electron/pull/2586#issuecomment-767168455

> In addition to now showing the blank note at revision 0 we have a few other things to consider.
> * if there are more than 60 revisions for a note then the revision at the 0 index won't be a blank note and thus it will be a legitimate selectable revision
> * the position on the slider at the far right should be one more than the number of available revisions but it shouldn't select a revision; it should select "no revisions" and appear as the current working copy of the note. this may be content the server isn't aware of yet and it's not possible to "revert" to it, but rather "reverting to it" would imply cancelling the dialog without reverting to any given version

> The "current version" for example can be misleading, since most folks will consider the current version to be whatever they have written in their note but if I'm not wrong, the revision slider says it's "the most-recent version of the note we got from the server"

@dmsnell and @sandymcfadden please feel free to add to this if there's anything I missed from later discussions!

Start with the revision slider and dialog behavior described in the issue, then read the discussion in pull request #2586. Verify how revision 0, notes with more than 60 revisions, the far-right slider position, and the current working copy are represented. Done means each listed slider state is selectable or non-selectable as specified and reverting preserves the intended behavior.

Written by the indexing model from the issue text.